rxvt-unicode is a clone of the well known terminal emulator rxvt.
Rob Holland of the Gentoo Linux Security Audit Team discovered that rxvt-unicode fails to properly check input length.
Successful exploitation would allow an attacker to execute arbitrary code with the permissions of the user running rxvt-unicode.
There is no known workaround at this time.
All rxvt-unicode users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=x11-terms/rxvt-unicode-5.3"